The present invention relates to an improvement in an apparatus for measuring a concentration of a light-absorbing substance in blood which employs pulse photometry as its operating principle: such as a pulse oximeter or an apparatus for measuring a pulse dye-dilution curve.
Pulse photometry goes beyond a pulse oximeter and is currently employed as a pulse dye dilution method. This method is made commercially practical as an apparatus for measuring a cardiac output, a circulating blood volume, a blood plasma disappearance rate of indocyanine green (ICG), and ICG clearance by administering a dye called ICG into blood and determining the concentration of the ICG in blood. This method is described in detail in the following references: Takehiko Iijima, et al. Cardiac output and circulating blood volume analysis by pulse dye-densitometry. J Clin Monit 1997; 13: 81-89; Takasuke Imai, et al. Measurement of cardiac output by pulse dye-densitometry using indocyanine green. Anesthesiology 1997; 87: 816-822; and Takasuke Imai, et al. Measurement of blood concentration of indocyanine green by pulse dye-densitometry-Comparison with the conventional spectrophotometric method. J Clin Monit 1998; 14: 477-484.
Further, the pulse dye dilution method is also applied to measurement of the concentration of abnormal hemoglobin, such as carboxyhemoglobin or methemoglobin, the concentration of hemoglobin, or the glucose level (see e.g., Japanese Patent Publication No. 3-71135B corresponding to U.S. Pat. No. 5,127,406 and Japanese Patent Publication No. 2002-228579A corresponding to U.S. Pat. No. 6,415,236).
Conventionally, for instance, when the concentration of a certain substance in blood is measured through use of two light beams having different wavelengths, the ratio Φ12 between variation in the attenuation of one wavelength and that in the attenuation of the other wavelength, the variation stemming from pulsation of blood, is determined. The concentration of the substance is calculated on the basis of the phenomenon that a certain constant relationship exists between Φ12 and the concentration of the substance (see e.g., Japanese Patent Publication No. 53-26437B). Specifically, the concentration of the substance is expressed as:C=F(Φ12),where C denotes the concentration of a substance in blood and F denotes a function representing a constant relationship.
In general, when “n” light beams having “n” different wavelengths are used, there are used, at most, “n−1” of attenuation variation ratios Φ of the respective wavelengths. For instance, if the light beams have three wavelengths, the concentration of a substance is expressed as:C=F(Φ12,Φ13)through use of a ratio Φ12 between variation in attenuation of a first wavelength and that in the attenuation of a second wavelength and a ratio Φ13 between variation in attenuation of the first wavelength and variation in attenuation of a third wavelength.
In the case of a pulse oximeter, the concentration C of a substance in blood is expressed as oxygen saturation in arterial blood SpO2 (a ratio of oxyhemoglobin concentrations to hemoglobin concentrations; that is, O2Hb/Hb). In the case of pulse dye-dilution curve measurement instrument, the concentration C of a substance in blood is expressed as a ratio of dye concentrations Cd to hemoglobin concentrations Hb; that is, a ratio of Cd/Hb.
However, according to such a measurement method, an approximately constant relationship exists between the concentration of a substance and the attenuation variation ratio. However, the relationship involves an individual difference. Even in the case of a single individual, the relationship varies according to a time point when measurement is performed or a measurement location, and variations are responsible for an error in measurement. For instance, in the case of a pulse oximeter, a calculated value varies by about 1% as a result of changing an attached probe from one finger to another finger or raising/lowering a hand, provided that an actual oxygen saturation in arterial blood SpO2 is constant. The following are conceivable as leading causes of the measurement error.    (1) Since blood has a light scattering nature, an attenuation derived from scattering varies depending on the thickness of blood.    (2) Two light beams are present; that is, a light beam passing through blood and another light beam not passing through blood.
When the concentration C of a light-absorbing substance in blood is determined through use of pulse photometry in the previously-described manner, a function taking, as a variable, only the attenuation variation ratio Φ has hitherto been used. Therefore, no consideration has been given to the dependence of an attenuation derived from scattering on the thickness of blood (not a thickness corresponding to a change but the overall thickness of blood). Further, there exist a light beam passing through blood and another light beam not passing through blood (i.e., a light beam passing through only a living tissue other than blood). Hence, no consideration has been given to the light beam not passing through blood, which in turn causes an error.